The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon.
1. Field of the Invention
This invention relates generally to a system and a method for measuring a quantity of NO2 in a gas sample. The invention relates more specifically to a system and a method which employ ultraviolet light to effect the photolytic dissociation of NO2 to NO.
2. Description of Related Art
Improved understanding of human-induced and natural atmospheric chemistry requires a sensitive and specific measurement of NO2, a molecule which is a key species in atmospheric ozone formation and loss processes. An ideal measurement of NO2 would be inexpensive and simple to operate, while providing quality data at high time resolution.
Most conventional commercially available instruments used for measuring NO2 in the atmosphere employ hot metal catalysts for NO2 conversion. These conventional devices, however, are not specific for NO2.
For example, one commercially available converter design is based on the reduction of NO2 to NO on a heated substrate (i.e., thermal decomposition), typically molybdenum oxide or, less of ten, ferrous sulfate. These surface-based converters are not specific for NO2, as they also efficiently reduce other atmospheric nitrogen-containing compounds to a detectable form. (Fehsenfeld, F. C., et al., Intercomparison of NO2 measurement techniques, Journal of Geophysical Research, 95, 3579-3597, 1990; Fehsenfeld, F. C., et al., Ground-based intercomparison of nitric acid measurement techniques, Journal of Geophysical Research, 103, 3343-3353, 1998.) Use of these converters can result in a gross overestimate of ambient NO2.
Another technique, the photolytic dissociation of NO2 with UV light, followed by chemiluminescence detection of the product NO, has been employed for ambient measurements of NO2 for over two decades. (Kley, D., et al., Chemiluminescence detector for NO and NO2, Atmospheric Technology, 12, 63-69, 1980.) This broadband photolysis technique has provided field measurement data used to evaluate and improve the current understanding of tropospheric and stratospheric ozone chemistry, radiative transfer, and sources and fate of reactive nitrogen compounds. The photolysis-chemiluminescence (P-CL) technique has been compared to other NO2 measurement techniques on the ground (Mihelcic, D., et al., An improved method of measuring tropospheric NO2 and RO2 by matrix isolation and electron spin resonance, Journal of Atmospheric Chemistry, 3, 341-361, 1985.; Fehsenfeld et al., 1990) and aboard aircraft (Del Negro, L. A., et al., Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, Journal of Geophysical Research, 104, 26, 687-26, 703, 1999), and been shown to provide useful data over a wide range of concentrations, ambient environments, and integration times.
NO2 is photodissociated at ultraviolet (UV) wavelengths below about 420 nm in a first-order process,
NO2+hxcexdxe2x86x92NO+Oxe2x80x83xe2x80x83(1)
with the rate constant for photolysis given by j (units of sxe2x88x921), which is the wavelength-integrated product of the photon flux (photons cmxe2x88x922sxe2x88x921), the weakly temperature-dependent NO2 absorption cross-section (cm2 moleculexe2x88x921), and the quantum yield for photodissociation (molecules photonxe2x88x921)(DeMore, W. B., et al., Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling, NASA Jet Propulsion Laboratory, Pasadena, Calif., 1997). In air, the O atom formed in (1) reacts rapidly with molecular oxygen to form O3:
O+O2xe2x86x92O3xe2x80x83xe2x80x83(2)
which can then react with NO to re-form NO2:
NO+O3xe2x86x92NO2+O2xe2x80x83xe2x80x83(3)
with the second-order rate constant for (3) given by k (cm3 molecule-""sxe2x88x921). During the daytime in the atmosphere, a photostationary state (characterized by zero net concentration change occurring over time) is established via these coupled reactions (Leighton, P. A., Photochemistry of Air Pollution, Academic Press, New York, 1961). Under daytime conditions, a new photostationary state will be established within 1-2 minutes of a perturbation to j or to the concentrations of the chemical species listed above.
Significant changes to concentrations of these coupled species can therefore occur during measurement, a result of perturbing the j value when ambient air is sampled into an instrument (Butcher, S. S., et al., Effect of inlet residence time on analysis of atmospheric nitrogen oxides and ozone, Analytical Chemistry, 43, 1890-1892, 1971; Bollinger, M. J., Chemiluminescent measurements of the oxides of nitrogen in the clean troposphere and atmospheric chemistry implications, Doctoral thesis, University of Colorado, Boulder, Boulder, 1982; Ridley, B. A., et al., NO and NO2 in the troposphere: technique and measurements in regions of a folded tropopause, Journal of Geophysical Research, 93, 15, 813-15, 830, 1988; Parrish, D. D., et al., Systematic variations in the concentration of NOx (NO plus NO2) at Niwot Ridge, Colorado, Journal of Geophysical Research, 95, 1817-1836, 1990). This occurs despite the minimal surface loss on most materials exhibited by these species. If total instrument sample residence times, from inlet tip to detector, are greater than a second or so, non-negligible bias in the derived concentrations of NO, NO2, and O3 can result from reactions (1) through (3) occurring during sampling.
The presence of other ambient oxidants (e.g., HO2 or RO2 species), or the occurrence of surface-induced oxidation of NO (Ridley et al., 1988), act to increase this bias. Data reduction procedures have been developed to account for reactions (1) through (3) during sampling and are relevant to all NO, NO2, and O3 measurements except open-path designs [Kley et al., 1980; Bollinger, 1982; Ridley et al., 1988; Parrish et al., 1990). These procedures were developed assuming pseudo-first-order conditions, i.e., that ozone is in large excess relative to NO and NO2, and that peroxy radical concentrations are negligibly small. These assumptions do not necessarily apply in many urban areas and in power plant plumes, as indicated in the NO2 Data Reduction section, below.
Reaction (1) is exploited in the P-CL measurement to photodissociate NO2 to NO, and the resulting product NO is measured as an increase in chemiluminescence signal above that from ambient NO (Kley et al., 1980). Ambient NO2 concentrations are derived from the difference between two signals, both of which can be large and vary quite rapidly under changing atmospheric conditions. Efficient conversion of NO2 to NO serves to maximize that difference and improve instrumental sensitivity for NO2.
In sampled ambient air, the effective conversion fraction (CF) of NO2 is given by (Bollinger, 1982),
CF=[jxcfx84/(jxcfx84+k[Ox]xcfx84)]*[1xe2x88x92exp(xe2x88x92jxcfx84xe2x88x92k[Ox]xcfx84)]xe2x80x83xe2x80x83(4)
where j is the wavelength-integrated product of the NO2 absorption cross-section, the light source flux, and the quantum yield for photodissociation; xcfx84 is the sample residence time in the photolysis cell (Kley et al., 1980). The light source flux, and thus j, is determined by the choice of lamp, reflector and filter optics, and cell geometry. Here k[Ox] denotes the rate coefficient and concentration of any oxidant that reacts with NO to produce NO2 in the cell.
Examination of (4) shows that increasing j without increasing r is the most effective way of maximizing instrumental sensitivity to NO2. This is illustrated graphically in FIG. 3, which shows CF calculated from (4) at 298 K as a function of cell residence time and ambient ozone concentration for j values ranging over a factor of nine. Higher j values confer the additional benefit of decreasing CF dependence on variations in ambient oxidant levels (FIG. 3). Further, the magnitude of the correction for changes in NO, NO2, and O3 concentrations during sampling is reduced at higher j values (e.g., Ridley et al., 1988).
Equally important is that (4) holds during sample transit in inlet tubing, where jxcx9c0 (Butcher et al., 1971). Correctly accounting for loss of NO and O3 and the formation of NO2 during the entire sampling process, including transit through inlet lines, is critical for accurate retrieval of ambient NO, NO2, and O3 mixing ratios from measured data. Neglect of reaction (3) in data analysis results in a systematic error in the determination of NO2; this error is dependent on ambient oxidant levels and ranges from 10 to 20% at sample residence times of 3 seconds or more. The sampling issues mentioned above can be significantly minimized by decreasing instrumental sample residence times (Ridley et al., 1988). Thus, increasing j and minimizing xcfx84 in the inlet and photolysis cell dramatically improves instrument accuracy and simplicity of data reduction in P-CL instruments.
Additional benefits of short sample residence times come in the form of increased instrumental time resolution and potentially enhanced specificity for NO2. Time resolution in well-designed P-CL systems is limited by longitudinal difflusion and mixing in the photolysis cell. Undesired sample mixing during longer cell residence times acts as a low-pass filter on high-frequency variations in ambient NO2, so that peak amplitudes are attenuated and information is lost (Ridley et al., 1994). Short instrumental sample residence times tend to minimize unwanted conversion of other nitrogen-containing species to detectable forms (e.g., Gao, R. S., et al., New photolysis system for NO2 measurements in the lower stratosphere, Journal of Geophysical Research, 99, 20, 673-20, 681, 1994). Operational photolysis systems represent compromises between conversion efficiency, time resolution, and specificity for NO2.
Both broadband and monochromatic light sources have been used to effect photolysis of NO2. For example, in a broadband technique, U.S. Pat. No. 3,840,342 describes a method of converting NO2 to NO which includes maintaining the NO2 at a temperature from about 40xc2x0 C. to about 130xc2x0 C. while exposing the NO2 to ultraviolet radiation.
The spectrally narrow output of XeF (353 nm) or Nd:YAG (355 nm) lasers have been used in an aircraft NO2 instrument (Sandholm, S. T., et al., An airborne compatible photofragmentation two-photon laser-induced fluorescence instrument for measuring background tropospheric levels of NO, NOx, and NO2, Journal of Geophysical Research, 95, 10155-10161, 1990; Bradshaw, J., et al., Photofragmentation two-photon laser-induced fluorescence detection of NO2 and NO: comparison of measurements with model results based on airborne observations during PEM-Tropics A, Geophysical Research Letters, 26, 471-474, 1999). While offering potentially large increases in specificity and sensitivity, laser photolysis systems are not yet widely utilized due in part to their relative complexity, size, weight, and cost.
At present, standard photolytic designs typically utilize the UV output of a collimated 300W or 500W direct-current (DC) short-arc Xe lamp to effect conversion (Kley et al., 1980; Ridley et al., 1988). Lamp ignition is accomplished by a very high (25 kV) voltage pulse, although the operating voltage is much lower (12-15 v DC). The short-arc Xe discharge approximates a point source, and emitted light is easily collimated by a rear parabolic reflector integral to the lamp body. Optimized systems using these lamps have reported a photodissociation rate constant (j value) of xcx9c0.37 sxe2x88x92, providing a calculated CF=0.31 in a cell residence time of 1 s (Kley et al., 1980). Longer cell residence times are typically chosen, affording increased NO2 sensitivity at the expense of instrument time response; at xcfx84=4 to 5 s, these systems exhibit a CFxcx9c0.50 (Parrish et al., 1990; Ridley et al., 1994). Useful power output between 320 and 420 nm is about 3 W, or 1% of the total power dissipated by the lamp (Gao et al., 1994).
These lamps emit strongly in the infrared (IR) region, and dielectric mirrors are always used to prevent IR wavelengths from entering the cell. In many designs the photolysis cell is cooled below ambient temperatures to further reduce interferences from thermal decomposition of other nitrogen-containing species during sampling. Published reports of measured conversion fractions for NO2 range from 0.25 to 0.6 in cell residence times of 2 to 5 s, corresponding to an effective j value approaching 0.2 sxe2x88x921. Complete P-CL instruments utilizing 500W or 1000W Xe lamps are marketed commercially (Eco-Physics, Ann Arbor, Mich.).
More recently, a P-CL system based on a 400W DC long-arc metal-halide lamp has been used to provide NO2 measurements (Gao et al., 1994; Gao, R. S., et al., Partitioning of the reactive nitrogen reservoir in the lower stratosphere of the southern hemisphere: Observations and modeling, Journal of Geophysical Research, 102, 3935-3949, 1997; Del Negro et al., 1999). Several improvements over the Xe lamp systems were demonstrated. Lamp ignition voltage is much lower, between 1 and 2 kV, and the operating voltage is roughly 120 v AC. Nearly 65W, or 16% of total power dissipated is emitted at useful (320 to 400 nm) wavelengths. Optical filters in the beam path are used to minimize undesirable thermolytic and photolytic conversion of other atmospheric species to detectable forms; the optical filters also attenuate roughly 40% of usable light. With filters in place, conversion fractions of 0.56 in cell residence times of 1.1 s were achieved. This system is characterized by an effective j value of 0.75 sxe2x88x921, which is a significant improvement over a Xe system of equivalent wattage (Del Negro et al., 1999).
The metal-halide lamp source is diffuse, approximating a line source, and requires an external dielectric mirror to reflect emitted UV light into the cell. The physically large photolysis cell (i.e., 5 cm i.d.xc3x9725 cm long) is well-matched to the lamp-reflector combination, and a 1.1 s sample residence time is obtained by reducing the cell pressure to xcx9c30 Torr. The lamp electrodes, optical filters, and cell are cooled by forced ambient stratospheric air initially at xe2x88x9250xc2x0 C. Without active cooling, the filter set would be destroyed by thermal stresses induced by the intense IR emissions from the lamp (Gao et al., 1994).
There are, therefore, size, cost, and thermal management issues attendant to conventional higher-wattage Xe or metal-halide arc lamp systems.
Therefore, a general need exists to provide a system and a method which employ ultraviolet light to effect the photolytic dissociation of NO2 to NO in a simple and efficient manner. A more specific need exists for a system and a method which overcome the aforementioned size, cost, thermal management, and data retrieval at high time resolution issues associated with conventional technologies.
It is an object of the present invention to provide a system and a method which employ ultraviolet light to effect the photolytic dissociation of NO2 to NO in a simple and efficient manner. It is a further object of the present invention to provide a system and a method characterized by i) higher conversion efficiency at faster time response; ii) lower power consumption; iii) less heat output with consequently less sample heating; iv) optically filtered light output for NO2-specific conversion, and v) simplified data reduction.
The present invention provides an efficient, lightweight, low-power NO2 photolytic converter suitable for laboratory, ground-, and aircraft-based field measurements. Conversion of NO2 to NO is accomplished by focusing the output from a Hg arc lamp into a photolysis cell maintained at subambient pressure. Recovery of ambient data is facilitated by minimizing and matching NO and NO2 instrument sample residence times in this multi-channel instrument, so that ambient NO2 may be easily retrieved at high time resolution. Specificity is enhanced by optically filtering the lamp output to minimize unwanted conversion of other ambient species characterized by photolysis cross-sections significantly different from NO2. Optical filtering also greatly reduces the spurious (or artifact) signal when sampling NO2-free air.
Accordingly, in a first preferred embodiment the present invention advantageously relates to a single-channel photolysis system for measuring a quantity of NO2 in a gas sample. The single-channel photolysis system comprises (a) a UV light source for emitting light capable of photolytically dissociating NO2 in the gas sample to NO; (b) a means for positioning the light source; (c) an ellipsoidal reflector for collecting and focusing the light from the light source; (d) an enclosure for enclosing the light source and the ellipsoidal reflector; (e) an optical filter assembly for receiving, filtering, and transmitting the focused light from the ellipsoidal reflector; (f) a shutter capable of blocking the transmission of the filtered light which is transmitted through the optical filter assembly; (g) a sample photolysis cell for containing a volume of the gas sample; (h) a means for controllably introducing the gas sample to the sample photolysis cell, and a means for controllably delivering the gas sample from the sample photolysis cell; (i) a detector capable of detecting an amount of the NO present in the gas sample delivered from the sample photolysis cell, and capable of emitting a signal representative of the amount of NO; and (j) a means for measuring the signal so as to quantify the amount of NO.
The invention further relates to a method of measuring a quantity of NO2 in a gas sample with the aforementioned single-channel photolysis system. In a typical embodiment, the method comprises the steps of (a) introducing a first portion of the gas sample to the sample photolysis cell; (b) irradiating the first portion of the gas sample with the light capable of photolytically dissociating NO2; (c) delivering the first portion of the gas sample from the sample photolysis cell to the detector so as to detect the amount of NO (comprising ambient NO and that NO formed by photolysis of ambient NO2) present in the first portion of the gas sample, and so as to emit the signal representative of the first portion amount of NO; (d) measuring the signal so as to quantify the first portion amount of NO; (e) positioning the shutter so as to block the transmission of the filtered light to the sample photolysis cell; (f) introducing a second portion of the gas sample to the sample photolysis cell; (g) delivering the second portion of the gas sample from the sample photolysis cell to the detector so as to detect the amount of NO present in the second portion of the gas sample, and so as to emit the signal representative of the second portion amount of NO; (h) measuring the signal so as to quantify the second portion amount of NO; and (i) positioning the shutter so as to allow the transmission of the filtered light to the sample photolysis cell.
In a second preferred embodiment the present invention advantageously relates to a dual-channel system for measuring a quantity of NO2. The dual-channel photolysis system comprises a gas inlet line for receiving a total gas sample; a flow divider for dividing the total gas sample into a first gas sample having a first gas sample volume, and a second gas sample having a second gas sample volume, the first gas sample volume and the second gas sample volume being equal; a first channel for detecting ambient NO in the first gas sample; and a second channel for detecting both ambient NO, and NO resulting from the photolytic dissociation of NO2 to NO, in the second gas sample.
In the dual-channel system, the first channel comprises (a) an opaque sample cell; (b) a first channel means for controllably introducing the first gas sample to the opaque sample cell, and a first channel means for controllably delivering the first gas sample from the opaque sample cell; (c) a first channel detector capable of detecting an amount of NO present in the first gas sample delivered from the opaque sample cell, and capable of emitting a signal representative of the amount of NO in the first gas sample; and (d) a first channel means for measuring the signal so as to quantify the amount of NO in the first gas sample.
In the dual-channel system, the second channel comprises (e) a UV light source for emitting light capable of photolytically dissociating NO2 in the second gas sample to NO; (f) a means for positioning the light source; (g) an ellipsoidal reflector for collecting and focusing the light from the light source; (h) an enclosure for enclosing the light source and the ellipsoidal reflector; (i) an optical filter assembly for receiving, filtering, and transmitting the focused light from the ellipsoidal reflector; (j) a shutter capable of blocking the transmission of the filtered light which is transmitted through the optical filter assembly; (k) a sample photolysis cell for containing a volume of the second gas sample; (l) a second channel means for controllably introducing the second gas sample to the sample photolysis cell, and a second channel means for controllably delivering the second gas sample from the sample photolysis cell; (m) a second channel detector capable of detecting an amount of NO present in the second gas sample delivered from the sample photolysis cell, and capable of emitting a signal representative of the amount of NO in the second gas sample; and (n) a second channel means for measuring the signal so as to quantify the amount of NO in the second gas sample.
In the dual-channel system, the first channel means for controllably introducing the gas sample, the opaque sample cell, the first channel means for controllably delivering the gas sample, and the first channel detector capable of detecting an amount of NO define a first channel gas flow volume. The second channel means for controllably introducing the gas sample, the sample photolysis cell, the second channel means for controllably delivering the gas sample, and the second channel detector capable of detecting an amount of NO define a second channel gas flow volume. The first channel gas flow volume, and the second channel gas flow volume, are minimal and equal.
The invention further relates to a method of measuring a quantity of NO2 in a gas sample with the aforementioned dual-channel photolysis system. In atypical embodiment, the method comprises the steps of, in the first channel, (a) introducing the first gas sample to the opaque sample cell; (b) delivering the first gas sample from the opaque sample cell to the first channel detector so as to detect the amount of NO present in the first gas sample, and so as to emit the signal representative of the amount of NO in the first gas sample; and (c) measuring the signal so as to quantify the amount of NO in the first gas sample.
In the second channel, the method comprises the steps of (d) introducing a first portion of the second gas sample to the sample photolysis cell; (e) irradiating the first portion of the second gas sample with the light capable of photolytically dissociating NO2; (f) delivering the first portion of the second gas sample from the sample photolysis cell to the second channel detector so as to detect the amount of NO (comprising ambient NO and that NO formed by photolysis of ambient NO2) present in the first portion of the second gas sample, and so as to emit the signal representative of the first portion amount of NO; (g) measuring the signal so as to quantify the first portion amount of NO; (h) positioning the shutter so as to block the transmission of the filtered light to the sample photolysis cell; (i) introducing a second portion of the second gas sample to the sample photolysis cell; (j) delivering the second portion of the second gas sample from the sample photolysis cell to the second channel detector so as to detect the amount of NO present in the second portion of the second gas sample, and so as to emit the signal representative of the second portion amount of NO; (k) measuring the signal so as to quantify the second portion amount of NO; and (l) positioning the shutter so as to allow the transmission of the filtered light to the sample photolysis cell.
In the method of measuring with the dual-channel system, steps (a) through (c) are performed simultaneous with said steps (d) through (l).
Use of a 200W Hg lamp in the system provides conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate (PAN) conversion fraction of less than 0.006 and negligible conversion of nitric acid (HNO3). Unwanted (i.e., artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters.
Fast instrument response is attained by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in the dual-channel instrument, so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the system. This configuration permits simple and accurate retrieval of NO2 concentrations in conditions marked by extreme atmospheric variability, where ambient NO concentrations can change over several orders of magnitude in seconds. The system has been demonstrated to be more efficient, more sensitive, less subject to interferences, and simpler than previous photolytic designs. In addition, the system is much less expensive to purchase and operate than conventional designs.
In summary, the advantages associated with the embodiments of the present system and method are numerous. Generally, the invention facilitates more efficient, more specific, and simpler NO2 detection, with significantly improved time response, as compared to previous P-CL designs.
First, the system enables the use of a short-arc high-pressure Hg lamp. This provides approximately a factor of 5 increase in UV output, and a factor of 4 decrease in undesired IR output, per watt of power dissipated relative to a standard Xe lamp. Increased efficiency permits significantly enhanced sensitivity, increased time resolution, and minimizes sample heating. As most of the useful power from the Hg lamp comes in a relatively narrow band of wavelengths centered around 365 nm, optical filtering provides increased specificity for NO2 without undue reduction of conversion efficiency. In addition, the point-source character of the Hg lamps permits efficient collection of the emitted light.
Secondly, the system provides for focusing the lamp output. Enhanced efficiency is attained by focusing lamp output into the sample photolysis cell, thereby significantly increasing the photon flux (j value in Equation 4). This improves upon previous photolytic converter systems in which lamp output is collimated.
Thirdly, the system uses an ellipsoidal mirror external to the lamp. Locating the Hg lamp arc at one focus of a fast (i.e., f/2 or better) external ellipsoidal mirror collects nearly 80% of total emitted light. For comparison, a laboratory mirror and lens system using the same lamp collects only about 10-20% of the lamp output. The external ellipsoidal mirror is not subject to optical surface degradation from sputtered electrode material during lamp ignition and operation, is much less subject to distortion at typical operating temperatures, and permits easy realignment of the arc at the mirror focus. None of these advantages is afforded by the Xe lamps used in the conventional NO2 photolysis converters, which are characterized by internal parabolic reflectors integrated with the discharge electrodes.
Fourthly, the system changes the aspect ratio of the sample cell so as to increase time response. Use of a relatively small-diameter and long sample cell minimizes longitudinal diffusion during sample transit, preserving high-frequency variations in ambient NO2. Previous sample cell designs are matched to physically larger lamps and spot sizes, are characterized by larger diameter-to-length ratios, and result in more complete attenuation of high-frequency data.
Finally, the dual-channel embodiment of the system matches NO and NO2 sample flow paths in a two-channel instrument. Matching sample paths permits greatly simplified data reduction procedures, and significantly enhances the time resolution attained by the NO2 measurement. For the first time, NO2 measurements by P-CL can be obtained on timescales equal to or faster than the sample cell residence time. Improved time resolution, at 1 second or better, in the present system extends the measurement of NO2 to new areas, such as studying turbulent fluxes of NO2 and quantifying ambient concentrations in power plant emission plumes. Previous measurements made with conventional devices could only retrieve data on timescales longer than the sample cell residence time, and were subject to large uncertainties under conditions of high atmospheric variability.
The applications of the photolytic system are varied and numerous. The system can be used, in conjunction with an NO detector, as a specific measurement of gas-phase NO2 at concentrations ranging from parts per trillion to parts per million or higher. Current uses involve ambient atmospheric air measurements for research or regulatory purposes, the study of gas-phase reaction kinetics in a laboratory setting, and a wide variety of industrial process monitoring applications. A variety of potential future medical applications might involve the non-invasive monitoring of human breath for NO2.
Thus, the present invention provides a system and a method which are superior to the aforementioned conventional devices, because it is characterized by i) higher conversion efficiency at faster time response; ii) lower power consumption; iii) less heat output with consequently less sample heating; and iv) optically filtered light output for NO2-specific conversion.
The present invention has a much more efficient light source in the wavelength region of interest, and a much more efficient external reflective focusing system than any conventional system. This combination provides the key feature of high NO2 conversion efficiency at high time resolution.